This invention relates to an image reading apparatus for scanning and optically reading a document image and outputting the scanned indicia as electronic data. More particularly, this invention relates to an image reading apparatus equipped with two optical reading means for reading images on the front and back surfaces of the documents or sheets fed one at a time from a document supply tray by an automatic document feeder commonly known as an ADF.
Image reading apparatuses, normally called scanners, optically read the image of a document and convert the image into electronic data and transmit it to image forming apparatuses, such as personal computers, copy machines or facsimiles. Image forming apparatuses are also normally equipped with the following types of image reading apparatuses.
Image reading apparatuses have reading means for reading the images on the documents or sheets transported one at a time from a document supply tray by an automatic document feeder. Recent image reading apparatuses have been proposed to have a function using two individual reading means to read the images on the front and back surfaces of the document sheets fed one at a time by an automatic document feeder located on one document transport path without locating the sheet in a switch-back path.
Generally, such an image reading apparatus employs an optical reduction reading means that has a depth of focus of approximately 10 mm to read the indicia on one surface of a document and a contact image sensor that has a depth of focus of approximately 0.6 mm to read the other surface of the document.
Examples of the image reading apparatuses that are equipped with two reading means to read indicia on the front and back surfaces of a document are Japanese Patent Publications (KOKAI) No. 9-46484, No. 3-240361 and No. 11-136444.
In Japanese Patent Publication No. 9-46484, disclosed is a reading apparatus interposed between transfer rollers to transfer documents or sheets between two reading means. However, when such a basic configuration is employed, the apparatus becomes larger, and because the ADF can not be opened or closed, means for placing and reading a thick book type document can not be placed at the same time.
Japanese Patent Publication No. 3-240361 discloses a reading apparatus including optical reduction reading means to read the bottom surface of a document or sheet, and reading means of a contact image sensor to read the top surface of the sheet, with individual transport rollers (23 and 24) established at the reading positions of the respective reading means to press the sheet toward each of the reading means.
Japanese Patent Publication No. 11-136444 also uses optical reduction reading means to read the bottom surface of a sheet and reading means of a contact image sensor to read the top surface of the sheet. The transport roller (72) is located just before the reading position of the reading means above the document sheet transport path to feed the sheets. Further disclosed is an image reading apparatus provided with a backup roller (74) in a position opposing the image reading position of the reading means of the optical reduction for pressing the sheet toward the image reading sensor.
The image forming apparatuses of the past for reading images on the front and back surfaces of the sheet have all used transport rollers or backup rollers and have employed a configuration to transport the sheets so that they are pressed against the reading surface or contact glass of the respective reading means that are formed at opposite sides. This is because it is required to transport the sheets through gaps that have different depths of focus for the two types of reading means to enable precise reading of the sheets without the images becoming out of focus when using two individual reading means to read both front and back surfaces of the sheets transported by the automatic document feeder apparatus.
Of particular note, the contact image sensor reading means has a smaller tolerance for the reading space between the sheet and the reading means in comparison to the optical reduction reading means that uses a CCD. So, the tolerance of the gap between the sheet and the image sensor must be strictly maintained.
Japanese Patent Publication No. 11-32164 teaches a reading apparatus forming a transport path having in the contact image sensor reading means a depth of focus in the transport path gap for the reading units of both reading means in place of using transport rollers to press the sheet against the contact glass.
All of the image reading apparatuses of the three described in the prior art are complex in structure and are large because they use transport or backup rollers near the image reading apparatuses in the transport paths. Furthermore, because the sheets transported in such apparatuses receive the driving force in the direction of travel and the vertical direction pressing force while being sandwiched between the pair of the transport rollers, the pressure applied to the sheet is dramatically changed from the time when the document sheet is between the pair of the transport rollers and when released from between the pair of the transport rollers, particularly with thicker document or sheet. Thus, it is not always possible to maintain the gap in the tolerance range between the document sheet and the reading surface. Furthermore, depending upon the pressure of the transport roller, the contact glass can become dirty because the document sheet comes to contact with the glass under the pressure of the rotating outer surface of the roller. Such problems are particularly undesirable for the optical devices.
The image reading apparatus disclosed in Japanese Patent Publication No. 11-136444 has a configuration for feeding the document or sheet by the pair of transport rollers (72) to the reading positions of the contact image sensor and optical reduction reading means, so there is a possibility of the sheet flapping in the vertical direction thereby making it impossible to ensure that the sheet image can be read precisely.
Also, if the gaps are adjusted to be the same for the reading units for both reading means, the focus for reading with the contact image reading means will be incorrect when the depth of focus is set to the standard of the optical reduction reading means. On the other hand, if setting the standard on the depth of focus of the contact image reading means, as in Japanese Patent Publication No. 11-32164, the gap in the sheet transport path becomes narrower, causing the sheets to travel for a greater distance in such a narrow gap thereby increasing the possibility of the problems like document sheet jams.
To make the image reading apparatus lighter and more compact, it is necessary to arrange the reading position for reading one or front surface of a document sheet and the reading position for reading the other or back surface of the document sheet as closely as possible without separation.
In view of these points, the image reading apparatuses in the prior art are insufficient in ensuring stability in the transfer of both front and back surfaces of the document or sheet with respect to the reading means reading surface, reducing the number of components for forming such apparatus and simplifying the structure.
An object of the present invention is to alleviate the problems of the image reading apparatuses of the past and to provide an image reading apparatus that limits the use of transport rollers and backup rollers, simplifies the structure and employs a compact image reading apparatus while attaining precise image data of the images on both front and back surfaces of the document or sheet by ensuring the stable transfer of the sheet on the reading means reading surfaces in each of the gaps that have different depths of focus tolerances for the front and back surfaces of the sheet with respect to the image sensors.